IAEA-116
VIC Library
0« 000002 « 006673'
INTERPRETATION ENVIRONMENTAL ISOTOPE OF
HYDROLOGY
REPORT OF A PANEL SPONSORED BY THE
INTERNATIONAL ATOMIC ENERGY AGENCY AND HELD IN VIENNA,
24-28 JUNE 1968
A TECHNICAL REPORT PUBLISHED BY THE
INTERNATIONAL ATOMIC ENERGY AGENCY, VIENNA, 1970
INIS Microfiche Clearinghouse International Atomic Energy Agency Wagramerstrasse 5
P.O. Box 100
A-1400 Vienna, Austria
on prepayment of Austrian Schillings 40.00 or against one IAEA microfiche service coupon.
IAEA-116
NTiL ISO!
REPORT OF A PANEL SPONSORED BY THE
INTERNATIONAL ATOMIC ENERGY AGENCY AND HELD IN VIENNA,
24-28 JUNE 1968
A TECHNICAL REPORT PUBLISHED BY THE
INTERNATIONAL ATOMIC ENERGY AGENCY, VIENNA, 1970
FOREWORD
The following papers, oral contributions and discussions, were assembled informally by the Scientific Secretariat of the Panel Meeting on Interpretation of Environmental Isotope Data in Hydrology (Vienna, Austria, 24-28 June 1968).
They have not been edited formally, so that minor
changes have not been cleared with the original
authors. They are reproduced for informal dis-
tribution as they may be helpful to hydrologists
and- others interested in the application of en-
vieonmental isotopes in hydrologic studies.
I N D E X
Page
1. INPUT OF ENVIRONMENTAL ISOTOPES TO EYDROLOGICAL SYSTEMS
1.1 GENERAL PATTERNS OF DEUTERIUM AND OXYGEN-18 CONTENT OF THE PRECIPITATION
Summary of the contribution by W. Dansgaard 1 1.2 ENVIRONMENTAL ISOTOPE VARIATIONS IN THE PRECIPITATION,
SURFACE 13ATERS AND TREE RINGS IN CANADA
Summary of the contribution by R.M. Brown 4 1.3 DISCUSSION ON THE FACTORS AFFECTING THE ENVIRONMENTAL
ISOTOPE CONTENT OF PRECIPITATION 6 1.4 DISCUSSION OF THE VARIATIONS OF TRITIUM CONTENT
OF THE PRECIPITATION IN SCANDINAVIAN PRECIPITATION
By E. Eriksson 7 1.5 A GLOBAL SURVEY OF ENVIRONMENTAL ISOTOPE DATA
Summary of the contribution by C. Lewis Meyer 8 1.6 DISCUSSION ON THE ENVIRONMENTAL ISOTOPE
DATA COLLECTION NETWORK 8 1.7 THE RELATIONSHIP BETWEEN ISOTOPIC COMPOSITION
OF PRECIPITATION AND LYSIMETER PERCOLATES 10 1.8 DISCUSSION ON LYSIMETER STUDIES 13 1.9 THE"USE OF ENVIRONMENTAL ISOTOPES IN INFILTRATION STUDIES
Summary of the contribution by L. Thilo and K.O. Munnich 14 1.10 DISCUSSION 17 1.11 ENVIRONMENTAL TRITIUM IN SOIL MOISTURE
AND GROUND!?ATER IN DENMARK
Summary of the contribution by Lars Jørgen Andersen 18 1.12 DISCUSSION 20 2. ENVIRONMENTAL ISOTOPES IN SURFACE WATER AND GLACIOLOGICAL STUDIES
2.1 THE USE OF ENVIRONMENTAL ISOTOPES IN PRECIPITATION- INFILTRATION-RUNOFF RELATIONS
Summary of the contribution by T. Dincer 22 2.2 DISCUSSION 23 2.3 THE USE OF ENVIRONMENTAL ISOTOPES IN GLACIOLOGICAL STUDIES
Summary of the contribution by W. Dansgaard 24
2.4 DISCUSSION 25
Page 3.1 SOME PRACTICAL CONSIDERATIONS IN ENVIRONMENTAL
ISOTOPE APPLICATIONS IN HYDROLOGIC STUDIES
Summary of the contribution "by G.H. Davis 27 3.2 DISCUSSION 29 3.3 ON THE APPROACH TO HYDROLOGIC PROBLEMS
Summary of the contribution by A. Nir 30 3.4 ON THE APPLICATION OP ENVIRONMENTAL ISOTOPE
DATA IN GROUNDWATER RESOURCE DEVELOPMENT
Summary of the contribution by T. Harpaz 31 3.5 DISCUSSION 32 3.6. UTILIZATION OP ENVIRONMENTAL ISOTOPE TECHNIQUES
IN WATER RESOURCE INVESTIGATION AND DEVELOPMENT 33 Summary of the contribution by R. Ambroggi and D.J. Burdon 3.7 AN EXAMPLE OP THE COOPERATIVE TTORK WITH PAO
IN GROUNDWATER INVESTIGATIONS IN JORDAN
By G.H. Davis 34 3.8 DJTSCUSSION 3 5 3.9 THE USE OP ENVIRONMENTAL ISOTOPES IN KARST HYDROLOGY
Summary of the contribution by B.R. Payne and T. Dinper 37 3.10 DISCUSSION 40 3.11 CONSIDERATIONS ON THE GROUNDWATER MOVEMENT IN KARST FORMATIONS
WITH REFERENCE TO TOTES GEBIRGE MASSIF IN AUSTRIA
By J.G. Z8tl 41 3.12 DISCUSSION 41 MATHEMATICAL MODELS IN ENVIRONMENTAL ISOTOPE STUDIES
4-1 COMMENTS ON THE USE OF MATHEMATICAL MODELS IN INTERPRETING THE ENVIRONMENTAL ISOTOPE DATA
Summary of the contribution by A. Nir 43 4.2 DISCUSSION 47 4.3 ON THE CHARACTERISTICS OF RIVER BASINS
Summary of the contribution by E. Eriksson 49
4.4 DISCUSSION 52
Page 5. LAKE WATE5 BALANCE STUDIES
5.1 LAKE WATER BALANCE STUDIES WITE ENVIRONMENTAL ISOTOPES Summary of the contribution by T. Dineer 54 5.2 DISCUSSION • 55
6. THE USE OF CAEBON-14 IN QROPITO WATER STUDIES
6.1 ADJUSTMENT OF RADIOCARBON AGES OF GROUNDWATER BY MEANS OF 13(j/14c
By B. Hanshaw 56 6.2 DISCUSSION 62 6.3 TEE CAEBON-14 WATEE AGE DETERMINATION
By I . Wendt 63 6.4 DISCUSSION 65 6.5 CARBON-14 STUDIES MADE 31 THE INSTITUTE
OF GEOLOGY OF THE AQUITAINE BASIN
By P. Leveque 6§
6.6 DISCUSSION 6§
6.7 ENVIRONMENTAL ISOTOPE INVESTIGATIONS IN THE GRAZ BASIN, AUSTRIA
Summary of the contribution by J.G. Zb'tl and G.L. Meyer 6%
ANNEX List of Participants
- 1 -
INPUT OP TTfVIPOFTTT X I~O' OF*"S TO HTPP0L03ICAL CYS7" :S
1.1 G^np.iL
PATT^PP"™? 2TrT~?rj::
.-/ID OXYGT:-18c^T^i^r
^F TIT "P^CIPITiTIO:1. Summary of the contribution made by Prof. T. Dansgaard.During the hydrologic cycle, the stable isotoioic conroonents of the water are exposed to several fractionaticn effects. One of these effects occurs because the vapour Tessure of the light isotoric component, H2 ^"O, is higher than the heavier comronents HDO and H2 ^"O. Another inmortant effect is the molecular exchange between vrater in the liquid and gas phases which is a complicated process.
For several years IiZA has carried out, in cooperation with 110, a collection of the -nrecrnitation samples in a world-wide network
of about 110 meteorological stations. These samples have been measured for tritium, deuterium and oxygen-18 content, and the following
discussion is based on these data.
Fig. (l) shows the mean oxygen-18 content 'of the r.recir.itation in January. The values presented are the weighted means of several January samples.
High oxygen-18 values are found in the mid-Atlantic and along the equator. The water which evaporates from the oceans condenses in such a "-ray so that initially the oxygen-18 content of the nreci-
"oitation is Relatively high. As tfie condensation -proceed1: however, the water valour and consequently nrecinitation originating from iT;' is more and more depleted in oxygen-18. This is the main cause for the change in oxy£-en-l8 with latitude seen in Fig. (l). A noticeable feature is a broad tongue of high values in the northern Atlantic.
This, which is -probably an indication of exchange between the
atmospheric water vigour and the ocean r^rface is also su^-norted by the low tritium values ^erha-ns resulting- from molecular exchange.
However, the oxygen-l8 and tritium variation over the northern
Atlantic could also be correlated by the nrecipitable water over this region, which would tend to increase the oxygen-18 content of precini- tation while decreasing the tritium content.
The "inland effect" is also illustrated in the same figure.
In North America nrecirit.stion decreasing inland is accompanied by a decrease in oxygen-18 content of precipitation. The same trend is observed in South-America; South Africa and in
This effect is also observed over the other hi.~h mountain ranges such as the Andes and Eocky Mountains of the American continent, and the Alps and mountains in Norway in Europe.
Fig. 2 shows the mean oxy,.en-l8 content of the preci-nitaticn in July. The patterns are almost th.% same as tvose of January but show a shift of oxygen-18 content toward higher values. In the
I n d i m Ocean high values of oxygen-18 can be explained by the fact that monsoon nrec:irotation represents the first stare of condensation.
-sed in relative deviation with res-nect to SMO" .
Pig. 3 shows "the annual mean values for deuterium and oxygen-18 content of the precipitation for all the stations of the network.
The correlation is linear and the slope of the line is 6. In arid zones the scattering can "be explained partly by the kinetic effect introru ;ed • by the evaporating raindrops.
The location of the annual values on a line with si one S and a positive intercept is explained by the following process: The ocean water, when evaporated, produces a vapour which is lighter than the water vapour but which is in equilibrium with the ocean water.
The difference is larger for oxygen-18 than for deuterium. Also upon condensation the water vapour produces precipitation which is in equi- librium with the vapour phase. Thus, the stable isotope composition of the precipitation is slightly lower than the ocean water, the difference being larger for oxygen-l8 than for deuterium. When the process of condensation continues, both the water vapour and the precipitation com- position move along a line with slope 8, which is defined initially by the water vapour originating from the oceans. The intercept of the line with deuterium axis is generally found to be + 10$o. but in certain regions intercepts higher than +"lO$o have been found. The above
correlation is made among stations, but when monthly values of oxygen-l8 and deuterium for a given individual station are plotted versus each other, a different regression line, which does not necessarily have the slo-ne 8 nor the intercept + 10$o, is obtained.
Evaporation from water causes the enrichment of remaining water vjith res-nect tojstabl-e isotopes. When the initial isotopic conmosition of such waters is the same (as in raindrops), the evaporated waters are found on a (f. Q - ^L granh along a line passing by the conmosition of the original water with a slope less than 8.
The altitude effect was studied in Innsbruck (600 m ) , Austria, and a station 6 km from Inns-ruck, Hafelekar, which is 700 m higher.
The oxygen-18 content of the precipitation in June showed significantly lighter isotopic composition in Hafelekar precipitation. In these mid- Alpine regions precipitation is released from clouds nearly all at the
same altitude so there should not be an orographic type altitude effect.
Pig. 4 shows that the difference of the isotopic composition of the precipitation in Innsbruck and in Hafelekar is due to the evaporation.
The intermediate values are from Honeburg, which is about 400 m higher than Innsbruck. The line joining Hafelekar and Honeburg values has a slope lower than 8, which indicates evaporation.
Pig. 5 shows the oxygen-18 content of the precipitation in winter in the same region. As expected, they are much lower than the summer values- the difference between Innsbruck and Hafelekar is not systemati-
cally positive, as it is in summar precipitation. This does not mean that there was no evaporation, or rather sublimation, from the precipi- tation in fall but it is due to the fact that sublimation from the snow flakes cannot bring the same type of fractionation as that from raindrops. In a snow flake or crystal sublimation takes place layer by layer, without any mixing process.
Pig. 6 shows the so-called amount effect in Kinshasa (Leopoldville).
Congo precipitation which can also be used in hydrological studies.
The oxygen-l8 content of intense and long duration rains must be lower than the oxygen-18 content of less intense rains with short duration.
Monthly precipitation is also given at the lower part of the figure.
There is an antiphase correlation and a correspondence of maximum and
- 3 -
minimum values for the amount of the -precipitation and its oxygen -IS content. At higher latitudes the same amount effect is observed in summer as shown in Pig. 7 which shows the monthly precipitation and its oxygen-l8 content in Tokyo. The amount effect vanishes in winter due to the solid state of the precipitation.
In Dolar regions the altitude effect is much greater than at the lower latitudes because no fractionation by evaporation can take place from solid precipitation. At higher latitudes the altitude effect for oxygen-18 amounts to O.&foo per 100 meters of altitude difference.
At low and mid-latitudes the altitude effect is 0.2$o per 100 meters.
1.2 ENVIRONMENTAL ISOTOPE VARIATIONS IN Tff^ PRECIPITATION, SURFACE WATERS AND IK TREE RINGS TN CANADA. Summary of the contribution made by R.M. Brown.
1.2.1 Deuterium in Precipitation and Surface Waters
A survey of the deuterium content of precipitation and surface waters across Canada is under way. Precipitation from 15 precipitation stations and river waters from 100 sites are being analyzed every second month for a two year period. Measurements to date show that seasonal variability is not more than i 2 p^m in surface streams in spite of being up to - 10 ppm in precipitation. Fig. 8 shows concentrations observed
in December 1967* Concentrations are expressed relative to SHOW - 157*6 derived from recent absolute calibration work in our laboratory.
Main features of the distribution of deuterium across Canada ares (a) High concentrations in the east and west coastal regions with a
sharp drop inland from the west coast because of the Rocky Mountains (b) Low concentrations in northern Canada where temperatures are low,
and central Canada which derives its moisture from northern air masses.
(c) High concentrations in the Great Lakes where a long residence time affords opportunity for extensive evaporative enrichment.
,Figures 9 and 10 show deuterium concentrations along the drainage systems of central Canada and the Great Lakes - St. Lawrence River.
The latitude effect is apparent in the Saskatchewan Rivers - high deuterium in the South Saskatchewan River, low deuterium in the North Saskatchewan River. The concentration increases in Lake Winnipeg
because of the admixture of water from southern Manitoba originating in the Gulf of Mexico - Mississippi Valley air mass system. In the Great Lakes - St. Lawrence system, local precipitation and tributary streams have lower deuterium than the main stream indicating enrichment has occurred in the Great Lakes. An enrichment of 2 ppm occurs across Lake Erie. Lower concentrations are observed down the St. Lawrence*
River where the admixture of local drainage, much of it from Ithe nurtli.
dilutes the Great Lakes deuterium.
1.2.2 Storm-to-storm Variation of Tritium and Deuterium in Precipitation
Figure 11 shows the storm-to-storm variability of deuteriuto' and tritium in Ottawa precipitation,. 1961. An inverse correlation of.
the deviations of individual tritium and deuterium values from their res-nective mean curves is observed. Studies of air mass trajectories"
show that the high tritium and low deuterium values are associated wi"t"h"
air masses of northern origin, low tritium and high deuterium with air masses of southern origin.
1.2.3 Tritium in Tree Rings
The distribution of tritium in the annual growth rings of trees has been investigated as a possible means of learning the history of tritium deposition at sites for which -^ast data is not available.
The method involves mechanical separation of individual rings} chemical separation of cellulose, removal of any labile tritium (the three
hydroxyl hydrogens of the cellulose unit) by boiling in 0.5 F HC1 solution made up with tritium-free water, combustion of the dried cellulose , electrolytic enrichment and low level counting.
- 5 -
Trees growing in tritiated groundwater in the CTNL Liquid Dis-nosal Area were examined first to see if recent tritium was incor- porated in older wood. Figure 12 shows the pattern observed. Although the high tritium concentrations of recent years penetrated the tree moisture back to 1945> it was not incorporated in cellulose formed r>rior to the time of its a-n-oearance in groundwater of the area. This work gave the history of tritium dispersal in this area.
Somewhat in contradiction to the foregoing, we have not been able to obtain values less than 50 TU on 1920-30 wood grown at an
uncontaminated site. For the present, we have accepted this as a back- ground level which we subtract from the measured concentrations of more recent rings to deduce the tritium pattern of the thermonuclear period.
Figure 13 compares concentrations observed in a tree grown on well- drained, sandy soil at .Deep River with mean concentrations of Ottawa precipitation for the growth period of each year. Excess tritium
appears in the rings of I960, 61, 62, but evidence from a few Deep River precipitation samples indicates that some reactor tritium from CRNL
(6 miles downwind) was deposited at Deep River at this time.
1.2.4 Perch Lake Evaporation Study
It was reported that P.J. Barry and W.F. Merritt are making an extensive study of evaporation from a small lake at Chalk River.
Perch La^e has been uniformly labelled with ab'out 0.5 /uCi/l tritium as a result of reactor liquid waste disposals. Absolute measurements of evaporation can be obtained by observing HTO vapour profiles over the lake.- The site is fully instrumented for meteorological and hydrological
measurements to obtain evaporation data by the various conventional methods (e.g. pan evaporation, water and energy budgets) for comparison with the tritium results. It was suggested that it would be valuable to make deuterium measurements in the course of this work to learn more about D / H fractionation in natural evaporation.
1.3 Discussions on the factors affecting the environmental isoto-oe content of precipitation.
The most important "effects" to be used in hydrological work were the so-called "altitude", "inland" and "temperature" effects, due to the rather local nature of the most hydrological and hydro-geological problems. The basic relation to be studied was the temperature effect because this is t>robably the cause of the altitude and latitude effects.
Temperature however, is closely related to the moisture content of the air, i.e. the lower is the temperature the lower is the
capacity of the air to hold moisture. Therefore, the stable isotope content - temperature relation has an intermediate step, namely, the moisture content of the air. Air with a low moisture content is
expected to contain less deuterium and oxygen-18 because, starting from its production in oceanic environments it should have gone through several processes of condensation and consequently of depletion with respect to stable isotopes. On the other hand, the influence of tempera- ture on the value of the fractionation factor is a negligible secondary effect which would be difficult to detect even in the condensation of water in nature.
In arid regions in the lower latitudes, evaporation of the rain- drops during their fall is important, and affects the stable isotope content of the precipitation. The initial stable isotope content of the raindrot) can be modified considerably toward heavier valwes during its fall. Precipitation on the southern slopes of the Sierra-Nevada Spain and the-Taurus, Turkey,mountain ranges shows a significant altitude effect as well as some deviation from the equilibrium line.
A question was raised concerning the antiphase variation of the tritium and deuterium content of the Canadian precipitition. The cause of this antiphase correlation could be that the storms with high turbu- lence contain more stratospheric air with high tritium and low
deuterium content. However the meteorologic investigations, which were carried out together with these isotope studies, indicate that this variation could only be due to the different sources of mristure.
A more general explanation of this antiphase correlation between tritium and deuterium content of the precipitation (which is observed in almost every region of the world) can be made by considering the
moisture content of the atmosphere. Dry polar air masses are likely to contain moisture with high tritium content and low deuterium content.
On the other hand, oceanic moist air masses have low tritium content and high deuterium content, reflecting their immediate marine origin.
When such problems are studied it is important to start with factors, such as temperature, moisture content, and to also consider the origin of the air.
In many actual applications of the environmental isotopes,
meteorologists and hydrologists are interested in the time variations of the isotopes at a given locality. The negative correlation between the tritium and deuterium contents of the precipitation raises interesting questions, such as the mechanism of atmospheric vapour transport.
The fluctuations about the recession line probably represent the local effects. The overall effect can be obtained by the superposition of the flux or the vapour transport effect with the local effect. This might lead to *.he quantitative evaluation of the both, provided a proper model is chosen.
- 7 -
1.4 Discussion of the variations of tritium content of the precipitation in Scandinavian precipitation^ by E. Eriksson As there is only a limited number of precipitation stations where tritium has "been measured for more than several years it is necessary to correlate the tritium content of the precipitation in stations with short tritium records with neighbouring stations having longer records.
The tritium-precipitation network in Scandinavia is extensive and has operated collecting monthly samples since 1961* It is possible to calculate the annual mean values of tritium in the precipitation and doing so to remove the seasonal variation. In the present analysis the Stockholm (Huddinge) meteorological station, which belongs to the WMO/lAEA network, was used as a benchmark.
There are a number of other stations in Sweden where samples for tritium are collected and 10 of these stations have 6 years of tritium record as does Huddinge station. The ratio of the annual mean tritium
concentrations of these stations to the annual mean tritium concentrations at Huddinge were calculated. These ratios varied considerably, both from one place to another and one year to another. The lowest value was
0.64 and the highest was a little higher than I.50. The purpose of this study was to find the error of the estimate of the mean annual tritium content of the precipitation using the annual tritium data in a neigh- bouring station. This can be done by using the well-known analysis of variance technique with two ways of classification, which are classifi- cation by stations and classification by years.
•Such an analysis allows elimination of the variations among
stations and the variations among the years. The variability among years indicates broad changes in climate from year to year.
Table I shows the final analysis of variance. The estimates of variances made with respect to stations and years are very significant compared to the residual. This means that there is a difference of
tritium deposition from-one station to another, but even more significant is the fact that the deposition pattern of tritium varies considerably from one year to another.
Table I
Table of Analysis of Variance
Origin of variation D.F. Variance estimate f Among stations 9 0.2470 16.41 Among years 5 0.1829 12.11 Eesidual 45 0.0151
These changes affect all the stations in any region.
The remaining variability can be explained as changes which affect only some stations, not all of them. The standard deviation of an individual estimate is 12$, of which approximately 5% results from analytical errors.
If the above analysis were done on a monthly basis, the variability of
"f" values would be higher. These results agree well with the results obtained by the chemistry of precipitation.
1.5 A GLOB1L SURVEY OF ENVIRONMENTAL ISOTOPE DATA.
By G. Lewis Meyer The Data
Since 1961, monthly precipitation samples for the IAEA/TOO Isotopes-In-Precipitation Network hare been collected at more than 100 meteorological stations in 67 countries and territories. To give the global coverage needed for a representative basic data network, stations at Argentine Islands in Antarctica Thule in Greenland, Uaupes in the upper Amazon Basin and Ouargla' in the Sahara have been requested to cooperate. Pig. 14 shows the stations in the network.
Stationkeepers participating in the network collect and conrnosite total rainfall for each month for tritium, deuterium and oxygen-18
analysis and bottle half-litre and 20-ml samples of the composite for tritium and for deuterium-oxygen-18 analysis respectively. The deuterium- oxygen-18 samples are shipped to Copenhagen for measurement under a
contract from the IAEA with the Oersted Institute, University of Copenhagen.
Approximately half of the tritium samples and selected weather data from all network stations are sent to the IAEA Laboratory in Vienna for analysis.
The remainder of the tritium samples are forwarded to cooperating tritium laboratories in Canada, India, New Zealand, Sweden and the USA for analysis. Supplementary isotope data are received by the TA"EA from other tritium laboratories in fTance, Germany, Iceland, the United Kingdom and the USA.
The IAEA acts as a collection and coordinating agency for the isotope and meteorological data necessary for hydrologic investigations.
.It has published the data at regular intervals. Data now (1968) included for each monthly sample are tritium, deuterium and oxygen-18 content, type and amount of precipitation, and mean temperature and relative humidity. These data are now computer-compatible and are available upon request on magnetic tape in the Fortran format. All old isotope and meteorological data available will be reissued in yearly blocks in a format prepared by the computer* These will begin to appear late in 1968 and new data will be issued yearly.
Similar but less extensive networks for collection of river and ocean water samcles for isotope analysis have been established within the framework of the IHD. Two networks, one of 21 major river stations and the other of 11 ocean stationst collect regular monthly instantaneous sanmles. As the programming workload permits, the data will be integrated into a common computer storage system for an "isotope-in-water" data file along with the precipitation data.
1.6 Discussion.
Some questions raised in the discussions were*
Is the monthly sampling really necessary?
Would a weighted composite annual sample be satisfactory in environmental isotope studies in hydrology?
These questions, it was suggested, could be partly answered if the existing data are properly analyzed and interpreted, depending in part on the type of application of the environmental isotope data. Averaging is a good practice if done in the bottle. It was also pointed out that the theory of sampling could supply part of the answer on the frequency of sanroling: If the frequency spectrum of the tritium output is considered it is seen that many fast molds are damped by the hydrologic system, so
- 9 -
th3t there is no point of collecting very frequent samples in the input.
However one should also be careful to collect environmental isotope data to handle unforeseen situations and problems which may arise in the future.
The use of environmental isotopes in hydrometeorological and groundwater studies requires different sampling methods. In hydro- meteorological work a high frequency of sampling the precipitation and broad areal coverage are necessary due to the high variability of the environmental isotope content of the precipitation both in time and space. On the other hand, the groundwater hydrologists normally require only the mean isotope in- ut to the groundwater systems. This can be obtained from the mean values for the isotope content of the precipi- tation if collected when the precipitation is closely related to the recharge to the groundwater. This is valid only in some regions where the precipitation is concentrated in a well defined period of the year (examples: mediterranean and monsoon climates). In Eimpe for example, the summer precipitation probably seldom reaches the groundwater and is largely transpired by plants. Thus the isotopic composition of only the winter precipitation, in such a case should be considered if a relation is sought between -the isotopic composition of the precipitation and the groundwater. Monthly sampling is needed,therefore, to estimate the input concentrations with more precision.
Quarterly sampling was also proposed as a compromise between the requirements imposed by the cost of the analysis and the reliability of estimates of the environmental isotope input concentrations to^the hydrologic systems*
1.7 THE RELATIONSHIP BETT/7EEK ISOTOPIC COMPOSITION OF PRECIPITATION" AND LYSIMETER PERCOLATES.
By. E. Halevy 1.7*1 Introduction
Realizing the importance of climatic and pedologic conditions on the quality and isotope composition of -mater in the unsaturated zone, the International Atomic Energy Agency set up a cooperative -programme to study the changes in and relationship between precipitation and soil water, as reflected in the composition of lysimeter percolates.
Lysiroeter stations were selected to represent a broad" spectrum of climates and soil conditions. This paper presents and discusses
results from lysimeter stations for which data accumulated for at least two years. Results of other stations will be published when more data becomes available. Although two years may not be enough for conclusive studies of this nature? the data accumulated so far have interesting implications which ,<may stimulate similar studies elsewhere.
1.7*2 Description of lysimeters and sampling methods
Each lysimeter was at least 2 x 2 x 2 meters (except Petzenkirchen), kept under natural regime (with one exception, Taastrup, discussed
later) including natural drainage (without suction).
Juprelle, Belgium (5O°41'N 5°26»E)j filled with a loess soil, texture clay loam, good drainage (krilium added to ensure good drainage). " «,
Mbl. Belgium (51°11 fIT 5°7fE)j sandy white soil. Excellent drainage. Data for this lysimeter are available for one year only. However, results are included for comparison with Juprelle and other stations.
Taastrup. Denmark (55°39'N 12°l8TE)j sandy soil covered with alfalfa. In summer 1967 the lysimeter was irrigated. Samples of irrigation water were taken and regarded as "precipitation"
for the purpose of the present study.
Coshacton, Ohio, USA (40°l6rtf 8l°5OTW)j constructed in a silt loam residual soil (Muskingum series, a Gray-Brown
Podzolic soil) on a slope of 23*2$. Good drainage, covered with pasture grass. Lysimeter Y101A was not fertilized whereas
Y101C was fertilized. In operation since 1944*
Petzenkirchen, Lower Austria (48°6 'If 15°12f!E)j size 1 x 1 x 1.3 m, filled with colluvial brown loam. Lysimeter 1 has no cover and developed spaces between well and monolith. Lysimeter 3 is grass covered. In operation since 1953.
Precipitation was collected on site, in a standard rain gauge.
Water collected during one month was combined for a weighted average.
The lysimeter percolates, with one exception, were collected in a
covered container, mixed at the end of the month for a weighted sample.
In Coshocton, the lysimeter data represent water collected on the last day of each month.
- 11 - Results and discussion
The results are shown in Pigs. 15 - 18. There are several common features to all data worth pointing out:
(a) Escept in Mol, a pure sand lysimeter. the isotopic conmosition of the percolates show a relative degree of stability, not reflecting the annual or seasonal variations.
(b) The weighted yearly average of stable isotopes in the percolate is depleted with respect to the precipitation.
(c) The tritium curves are receding- as a function of time, in a regular manner resembling the atmospheric recession.
(d) The weighted yearly averages of tritium concentration in the percolate is higher than that of the -precipitation.
Tritium Data
The main features of the tritium data are the recession trend and the difference "between the weighted averages of the ureci- pitation and percolation. Although the percolate tritium line shows in most cases a large decree of buffering caused by mixing and dispersion in the soil, the distinct recessive trend indicates vertical flow that keeps a temporal identity.
The degree of buffering and the turn-over time depend on soil texture and are more pronounced in loamy textured soils than in sandy soils.
Owing to the present moratorium on atmospheric nuclear
testing, tritium in the atmosphere is characterized by recession following the 1963-4 peak.
The higher than average concentration of tritium in the percolate in comparison with the precipitate indicates that the bulk of the percolated water is infiltration from the previous year or even earlier. The longest set of data on tritium is
provided by Petzenkirchen. Here, we find also the only exception>
1965 tritium values in the percolate are lower than the assumed average of the precipitation (the assumed value was reached "by comparing the yearly weighted averages Vienna and Petzenkirchen.
Vienna values in 1966 and 1967 were 20 TU higher. In 1965 the average for Vienna was 876 and thus a value of 850 was chosen for Petzenkirchen. This value seems to be reasonable from the Petzenkirchen data available for the period April-December 1965)*
If this peak is an actual reversal of the generally observed trend, it may belong to the peak year of the percolate activity and thus give us a rate of movement of 2 years because the
atmospheric peak of tritium in Vienna occurred in 1963* However, it is possible, as will be discussed from stable isotopes data, that computations based on yearly averages are not correct in all cases. The stable isotopes data, in cases where the precipi- tation - percolation ratio is high, are strongly biased towards winter precipitation. As observed, tritium values in winter
precipitation are generally lower than in spring and early summer*
The weighted average for the following winters (October-March) are:
year 1963/64 1964/65 1965/66 1966/67
•precipitation 1772 TU
577
303 208
•percolation
-
~ 750
~ 650
~ 400
These comparisons show further that at least a one-year delay occurs unless there is an immediate effective percolation of spring-summer precipitation which raises the average tritium concentration in the percolate. In Austria the summers of 1965 and 1966 had excessive rain falls. This may explain the
relatively high values for percolate 1965/66 which otherwise should be interpreted as originating prior to 1964/65* and the same may "be said for 1966/67 which show a pre 1965/66 value.
In Juprelle the weighted average for the percolate in 1967 is slightly lower than the precipitation of 1966 and thus
indicates a delay of about one year. In Taastrup the weighted averages of percolate and precipitate are close to each other and in 1966 they are equal. Thus, if the average for the whole year is considered,the turn-over time is less than one year. However, the sharp decline in composition of June 1967 caused by appli- cation of irrigation water which was followed by low values in July and August, does not show up in the percolate of the end of 1967 and thus the turn-over time may be'longer than half a year, but less than a full year.
Stable isotope data
Here too, in spite of the highly fluctuating input values, the output exhibits a large degree of stability. In no case does the deviation from the weighted mean exceed one delta unit for 1^0.
(Note: Deuterium values are not shown because they follow the same general pattern. However, the relationship between D and
"*-°0 is in some cases very useful and will be discussed separately).
Assuming that the years under consideration represent the normal climatic conditions, it is then possible to use the stable
isotopes data to compute the relative contribution of winter and summer input to the percolate since its average composition reflects the weighted mean of the effective infiltration.
For instance, the Coshocton data for the period 1966 and 1967 shows:
Weighted average of percolate -
11
» of winters (Jan/Mar, Oct/Dec)-
"
nof summers (Apr.-Sept.) - 5*7$o
Had the percolate consisted only of winter input, its compo- sition should have been different. If, however, we attribute
•70$ of winter input and 30$ of summer tlie actual composition iB
the outcome. As discussed above, there exists an average delay
of at least one year between input and output and thus such
calculations are only justified if the previous years were not
substantially different from 1966/67* An attempt to compare the
tritium data of Coshocton and Chicago in order to obtain past
- 13 -
records for Coshocton showed a poor correlation and could not be applied to this case. As a test the assumed 7O-3OJ»
effective input was applied to the 1966 tritium data in
Coshocton in order to predict the 196? percolate composition.
This resulted in a calculated value of 2*88 TD against the actual value of 313 TU. This may either be caused by a longer delay period (higher values in 19-65) or larger summer contribution.
The second assumption is not consistent with the stable isotope data. The same analysis for Juprelle shows an equal contribution of winter and summer. The application to tritium data results in a discrepancy of 40 TU (actual is lower than calculated).
In Taastrup- both tritium and stable isotope averages for input and output are close to each other and this type of calculation cannot be applied.
1.8 Discussion of the lysimeter studies.
It was reported that similar tritium output curves were obtained in lysimeter experiments made in Heidelberg, Federal Eepublic of Germany.
The considerable smoothing of the tritium in the lysimeter percolates was first attributed to mixing, but soil cores showed that there was no significant mixing of water layers having different tritium contents, although some smoothing due to diffusion did occur. The second effect, which is such more important, is the preferential intake of the preci- pitation falling during different periods of the year. The summer rains, with high tritium content, are more or less consumed by the plants and do not reach the lysimeter bottom. The same is true also
for late winter and spring precipitation. The early winter precipitation, which has relatively low tritium content, does reafih the lysimeter
/bottom. In the lower part of the lysimeters, the capillary fringe may also contribute to the smoothing of the tritium variations. The process of movement of soil moisture is a complicated one* the mean passage time denends on the moisture content and on the depth of the lysimeter.
The smooth curves of tritium which have values close to the annual mean tritium concentration in precipitation, are not due to the mixing they result from a process of selecting the precipitation which has a tritium concentration not very different from the annual mean.
Isotope data may provide valuable information on moisture movement in the soil, even in cases where basic hydrological data are missing. If a system is stable, and if deuterium and tritium contents of the precipitation and the percolate are available, information on the evaporation and mass transfer are not necessary.
The method of accounting for moisture can also be used to solve problems of soil moisture movement.-- In this method, however, the physical mechanism of the moisture transfer cannot be studied. It is possible to use both methods, i.e.,- moisture accounting and variations of the environmental isotopes in the input and output of the lysimeter study area. Certainly more information can be obtained in this way.
The major objective of the lysimeter studies was the investigation of changes in the isotopic composition of the water between its infil- tration into the soil and its percolation into the zone of saturation.
In this way the environmental isotope data of the precipitation can be used to estimate the isotope content of the input or recharge to
ground water systems.
Also reported were studies on large lysimeters (400 m surface area and 4 m deep). Three lysimeters having different plant covers are being studied with particular emphasis on C-14 analysis but samples are
collected also for analysis of tritium and deuterium content of the water.
Such studies require long periods of time because the water percolates very slowly in the lysimeters.
1.9 TEE USE OP ENVIRONMENTAL ISOTOPES IK INFHTFATIOH STODXBS.
BY L. Thilo and K.O. lEinnich.
H-3 and C-14 in shallow groundwater
The rise of both H and C concentrations in nature due to the 3 14 atmospheric tests of nuclear weapons opened the way for a number of useful applications in which these radioactive isotopes are used as
tracers. In particular, the way in which the increase of the 3 H (as well as that of 14c concentration) are reflected in shallow groundwater can be used to check the reliability of 3n
a nd 1 4 Q
ag
e g of groundwater.
Under favourable conditions bomb-produced tritium in uppermost ground- water also allows a determination of the recharge rate.
The validity of the isotope ages is based on a comparison of the bomb increase of both isotopes in the groundwater. An older age from 14c data than from % means that there has been exchange in the *
4C with carbonate compounds in the transit of water from the land surface to the groundwater sampling point. The average recharge rate, on the other hand, is determined by comparison of the total bomb tritium found in the
upper groundwater with the average tritium concentration in post-bomb rain. The -principles and the conditions for application and preliminary results in a few cases are given by Iffiinnich £~lJ7» With the technique described in detail elsewhere extensive sampling has been done in the area of Sandhausen (Ehinevalley near Heidelberg). A tritium profile taken at the same plaoe previously has been reported by Munnich £~lJ/»
At that time a specially drilled observation well in which** a filler tube had been blocked by rubber balloons, thus confining a certain depth interval, was sanroled. All tritium depth-profiles obtained are shown in Pig. 19. The total amount of tritium found below the water talfle (integral under TU vs. depth-curve) for the individual profiles is given in Table I.
Site, a a a b b b c d
Date
IX, 67XII,
67 I, 68 X, 66 TO, 67XII,
67 I, 68III,
68Table Sandhausen a, b, TU x meter/€
900 1160 1360 1220
980 >
1390 IJttO 1310
I
c and d
Depth of half value below water table
2.3 2.8 3.0 3.5
? 3.2 2.0 4-2
*) ' admixture from below during sampling
- 15 -
In Table I the variations with sampling site, as well as with the sampling date, are larger than was originally expected. Although the values for site A (Fig. 19s) seem to be rising with time, this increase is too rapid to be attributed 'to the continuous tritium input by
recharge. That recharge is not the cause of the increase can also be seen from the shape of the curves in Pig. 19a where only the upper part of the curves is pushed down whereas the lower part is similar.
The differences from one sampling site to another are not as serious as one might assume in view of possible inhomogeneities in the aquifer.
Although the sampling sites are quite near to each other (from 10 to 20 meters) small scale irregularities could still have an influence, since the flow velocity of the groundwater is only a few centimetres a day.
The slight indications of peaks or other details in the individual profiles may also be attributable to small irregularities in the aquifer. Although it is tempting to ascribe such peaks to the corresponding variations of tritium concentration in rain, it is
unlikely that such an effect would still be observed after the water has been flowing in the aquifer for almost ten years. Nevertheless, with the obtained recharge rate-of 170 mm/year and an assumed porosity of
£ e O.35> the variations of tritium-concentration in rain,if "projected"
into the aquifer are shown in Fig. 20. The sampling technique proved not to result in a vertical mixing of water in the aquifer even if larger samples are taken for ^4c measurement. ^See Fig. 19d}» To determine the recharge rate by measuring the tritium accretion in groundwater; it is necessary to take more than one tritium-profile to determine an average value for the tritium amount. From all the profiles measured in Sand- hausen an average value of (1200 ± 180) x £ - T U metre - below the water table is obtained. £ is the porosity. The standard deviation of 15%
is not too serious if compared to uncertainties in the porosity and moisture content above the water table.
The rise of H concentration in shallow groundwater can be conrDared with that of 14c. Since the laboratory experiments with aquifer material from Sandhausen indicate a surface-layer carbon exchange taking place between the solid and liquid phase, this should also occur in nature, as the material we used in the column was untreated. As was explained in ^"1 7- such an exchange would not influence equilibrium conditions where~the surface layer of the solid has been loaded to equilibrium with C. In the case of a rising 14c concentration in the water, however, the establishment of a new exchange equilibrium results in the same delay effect as observed in the column experiments. This effect can be sought by comparing the increase of ^H and 14c concentrations of groundwater at different depths.
Samples for C analysis were taken in the same way as samples for tritium. Since larger samples are needed (60 liters for groundwater of medium hardness approx. 200 m mole of COp are needed for the measure- ment) care must be taken to prevent vertical mixing in the aquifer during sampling. In the initial sampling technique with the moveable filter such mixing did occur, as has been shown by tritium measurement of the same samples (see profile of June 66 in Fig-. 1 9 b ) . With the new sampling technique (sampling downwards) no such mixing occurred
(tritium values in Fig. 1 9 d ) . The depth profile of 14c concentration
the uncertainty in € causes an additional error in the total amount of tritium in the groundwater.
in the groundwater is shown in Fig. 21. The profile for June 67 corresponds to sampling site of Pig. 19b and that for March 68 to Pig. 19d. The values for the June 67 profile for 5 and 6 metres might be higher, since the tritium values indicated admixture from below.
The ^ C values are indicated. As the undisturbed ^43 concentrations show, the initial value is around 60$ whereas the values normally found for groundwater are around 85$. This indicates that the input of CO^
into the groundwater happens in such a way that, after dissolution of CaCCL in the water containing CO^ the resulting HCO^ solution is no longer in contact with the CO- source. This is confirmed by the 13c values being around - ll$o (against PDB) instead of around - l6$o where after dissolution of CaCO, the water remains in contact with the
C0_ supply. This raises the ^t concentration from about 60$ to 85$
ana causes *^C to change from - 12$o to about - I6$o. A depth profile of the CaCO.. content of the aquifer in Sandhausen is shown in Pig. 22.
As there is^little CaCO, above the water table in the unsaturated zone, the C0_ can reach the water table in the gas-phase by diffusion.
It dissolves in the water causing dissolution of CaCO...
The exchange between the resulting HCO, solution at the water table and the C 0? gas phase is much less than it is between soil moisture and C0p in the unsaturated zone. This is due to (l) considerable diffusion resistance in the gas phase and (2) the absence of turbulence in the liquid. Thus, a liquid layer even of moderate thickness reduces diffusion to a minimum. Direct measurements of C0_ concentrations in the unsaturated zone made by J. Pantidis resulted, in values between - 0.3_and 0.5$ by volume. Such concentrations can keep about 3 m mo3fe HCO3 and 0.2 m mole CO- in solution. The total carbonate hardness of the water as measured from the samples increased from about 3 m mole just below the water table to nearly 5 ni mole 7*5 meters deeper
(see Pig. 23)» The &^C values of the CO,, above the water table being - 22 and - 20$o against PDB indicate the COp to be from plant origin.
The origin of the CO- is important when loosing for a delay of ^ C
increase in the groundwater. If from plant origin (humus), an additional delay of more than 2 years in the biosphere occurs. In Pig. 24 "the
14c increase in the atmosphere is "projected" into the groundwater with and without this delay of 2 years. Even without the two.years delay no delay of 14c incrase can be seen if compared to the C input.
With the half value-depth more than 3 meters below the water table, there also is no delay compared with the tritium profiles where the half value-de-nth is close to 3 meters below the water table also.
This result, on the other hand- should not be surprising if one thinks of the delay factor (see this report and £ 1 J) which is
calculated as /3= 1.1 for particle size 100 diameter, porosity O.35i CaCO.. content 15$ and carbonate hardness of 3 m mole/iitre. Even if this delay of 10$ occurs one would not expect to observe it.
Reference
£~1 7 K.O. Munnich et al., "Isotopes in Hydrology", p . 305 IAEA, Vienna (1967).
- 17 -
1.10 Discussion on the use of environmental isotopes in infiltration studies.
The first comment dealt with the horizontal groundwater movement.
Because of the continuity requirement, one has to consider the horizon- tal movement of groundwater and the modification it would imply on the tritium and carbon-14 profiles. TEhen the movement is slow, it does not distort much the profiles, as is probably the case in the aquifer
studied' the hydraulic gradient is 2foo and the groundwater velocity is estimated to be 20 m/yr.
A second comment pointed out that correction for the decay of tritium would improve the profiles presented in Fig. 19 and would lower tritium peaks. The method presented in the paper is based on the amount of tritium which goes to the groundwater rather than the identi- fication of thermonuclear tritium pulses. The former approach is more conservative than the second, which uses the shape of the tritium pro- files in the groundwater•
Similar profiles were observed also in two boreholes in the southern Vienna Basin. This suggests active recharge from above with high tritium content mixed with a horizontal contribution of groundwater flow that has a low tritium content. Although they give valuable
information on the rate of recharge such studies are much more compli- cated than anticipated*
In ^C measurements made in the U.S.A., in two wells 20 metres apart, one well going just below the water table, and the other
considerably below, at about 25 metres, the -^C data indicated the same age about 50$ of the modern standard for both wells. When ^C adjust- ments were made the ^4c of the shallow well came to 70$ and the
deeper one to &Ofo of the modern or more. This is rather a curious result as one expects younger water in the shallow well. Tritium results, however, confirmed the ^C results. The tritium content of the water sample collected from the shallow well was 20 TU- and from the deeper one was 50 TU. This indicated that the area around the shallow well receives recharge much less readily than the area around the deeper well.
Fig. 23 shows that water at-the water table has an appreciable carbonate content- this shows that solution has taken place in the non-saturated soil zone, a fact observed in other studies made in
Germany also.
1.11 EHYIF0BF3NTAL TEITIUM IN SOIL MDISTUEE AND GROUNDWAT^B IN DENMABK. Summary of the paper by Lars Jørgen Andersen.
Geogra-nhy and Geology
The investigations took place in a representative area of Karup in the central isart of Jutland Denmark (Fig. 2 5 ) . Two wells were augered at localities with different geological and hydrological
conditions. The first one Gr^nh^j no. 1 is in an unconfined aquifer of the outwashed sand and gravel of the last glaciation, which is the most common geological formation in the area. The land surface slopes slightly l,2^o. to WSW and the depth to the groundwater is about
25 m below land surface. Wiis well is located at the groundwater divide, the depth to water decreases toward a central stream which has a slope of about 1$
The second well is located near Engesvang in the southern part of the representative area, where the geological formations consist of the glacial drift, boulder clay from the Riss-glaciation and layers of melt- water sand. In the clay formation, a perched groundwater aquifer occurs from 5-10 m below land surface. Beneath this aquifer and the boulder clay is a water-table aquifer in the meltwater sand. The groundwater table here is about 25 m below land surface.
Drilling and sampling procedure
At two localities 6-inch wells were augered. Special care was taken to prevent waj;er from entering the hole during drilling. The material was sampled every*20 cm and stored in sealed tin containers for later evaporation tests the weight of each sample was about 6 kg.
Below the water table, water samples were*taken by hammering a screen, half a meter down below the bottom of the hole, and pumping by compres-
sed air. The groundwater was stored in glass bottles. The pumping was continued long enough'before sample collection to obtain raw ground- water. The soil moisture from the unsaturated zone was later evapora- ted by distillation at. 110 - 125°C« The Bamples were weighed before and after evaporation and the distilled water from the samples was weighted. The total loss in weight during the procedure averaged 1.8%.
Th© water loss can probably be exnlained as a result of the effects of a vacuun water pump whfch was introduced in.the pT©«edure to accelerate distillation.
The volume of evaporated water varied from one sample to another depending on the moisture content, which varied from 150 - 1500 cm3.
The number of samples, evaporated samples and analysed samples from each of the two wells are as follows:
N U M B E R O P S A M P L E S
j
Grj^nh^j no. 1
i fengesvang
Prom the unsaturated zone j Prom the saturated zone 1
sampled evaporated) analysed; sampled j analysed
j 1
110 ' 43 ; 32 ! 11
j ! ]-
90 j 24 j 8 ] 8
5 • \
0
1
- 19 -
The tritium analysis has been made by the Danish Isotope Centre in Copenhagen by Mr. E. Morck. The benzene method was used. The sensi- bility of this method is 10 TU and the standard error is -^/o. All the values of the tritium concentrations are given fox December 1967*
The tritium x>rofiles
The result of the tritium analysis is shown graphically on Pigs.
26 and 27 together with a geological log and a moisture log.
The moisture determinations were made by the neutron method.
No tritium determinations are available on precipitation samples from Danish stations during the previous years. Therefore, the values of Huddinge, Sweden, were adjusted with a factor of O.85 and corrected for radioactive decay up to December 1967*
Values of precipitation at Gr/nh^j and estimated values of the difference between precipitation and evapotranspiration from January 1962 - March 1966 together with the above-mentioned adjusted tritium values from Huddinge are tabulated in Table I, and weighted average of tritium concentrations for the periods October - December. September- March and the year from February - February are tabulated in Table II.
The low tritium content in the soil moisture below 18 m in the profile from Gr/nh^j may originate from precipitation from 1962 or earlier, and the three peaks may represent recharge from precipitation from the years 1963. 1964 and 1965* The low values in the upper two metres correspond to the precipitation values fj^pm October 1965 to March 1966.
If the calculated tritium concentrations for the precipitation at Gr^nhf$j are correct, the tritium concentration of tKl" soil moisture shows that the recharge may originate partly from precipitation in the summer time too. It can be seen from Tables I and II that the tritium concentrations of the precipitation from wintertime are too low during the years 1962 - 1965 to produce the registrated- tritium content of the soil moisture. From Table I it can be seen that an excess in precipi- tation (the volume of precipitation minus the volume of evapotranspi- ration) normally exists in all months except June and July. This should also indicate that the possibilities of recharge to the groundwater or to the deeper parts of the unsaturated zone is possible during most of the year in the investigated area.
In the profile from Engesvang (Fig. 27) the low content of tritium in the unsaturated zone below the perched aquifer indicates that only small amounts of water infiltrate to this depth. This may be due to low hydraulic conductivity of the clay formation, but the relatively high moisture content of the clay beds should also prevent a faster movement of the front of the infiltrated water. If the measured content of 30 TU from the groundwater level is correct, it can be
interpreted as a result of horizontal movements of groundwater and the origin of the tritium should be referred to as recharge outside the area of the perched groundwater aquifer.
The amount of the recharge
If it is assumed that all soil water successively will be replaced by the later infiltrated water it should be possible to determine the amount of recharge if the total amount of soil water and the time during which it has been infiltrated are known.
This should be the case at the well Gr^nh/j no. 1. Prom the moisture log the amount of the total stored water in the unsaturated zone can be calculated. A summation of soil moisture between ground surface and 18 m below ground surface gives a stored soil water volume of about 1600 mm.
In accordance with the above assumption, this volume should be equal to the recharge during the years 1962-1966 because the tritium profile shows that the recharge from 1962 has arrived to a depth of about 18 m below ground surface. The yearly amount of recharge as an average of these 4 years should be calculated to be about 400 mm.
The summation of excess of precipitation during the same period gives a value of about 1350 mm for the recharge. The deviation between these two values can be explained if the estimated values of the evapo- transpiration are 400 mm per year but perhaps this value is too high and a value of 350 mm would be more realistic. This would increase the excess in precipitation with 200 mm and agreement of the two determi- nations would be achieved.
Conclusion
The investigations have shown that the soil moisture is well
stratified in Tespect to tritium content and that a change in the content of tritium of the precipitation during a rather short time seems to be detectable in the soil moisture. It can be concluded that the summer precipitation also contributes to the recharge.
The tritium profile at Gr^nh^j shows that the infiltration rate in homogeneous formations is rather low (4-5 m/year) in spite of a
rather high recharge (300 - 400 mm/year) and a low soil moisture content (about 10 vol.$). Tritium profiles in the unsaturated and saturated zones would give much more information about the hydrological properties if the input concentration of the recharge water was better known.
This probably can be done by a continuous sampling of recharge water, just below the root zone by using tensiometres under vacuum.
1.12 Discussion on environmental tritium in soil moisture and groundwater in Denmark.
By means of ^-transmission methods in two drillholes, the high apparent speed of the moisture movement, compared to the actual movement of water is also observed in other experiments.
After a heavy rain moisture seems to move very fast downwards.
In summer the movement of moisture does not reach the groundwater table, it actually is sucked by the plant roots to be transpired and evaporated.
When the water content of the soil exceeds the specific retention
of the soil, the downward movement starts. This movement is faster when
the volume of the gravity water is larger. This type of movement is
different"from that taking place under saturated conditions where the
response is almost immediate. In the case presented in the working
paper the total porosity was 35$ and the moisture content only
10 ~ 15$ per volume.
- 21 -
Table I
Precipitation at Gr^nh^j, estimated values of excess (P-E ) and
corrected values of tritium content in precipitation at HuddYnge 1962-66
! {'
t
December November October September August July June May April March February January
P mm
56 56 32
1966 P-B^
mm
40 51 32
T
oorr TU3
213 204 196
P mm
92 54 44 68
62 0446 34 70-
2023 74
1965 P-E^
mm
87 39 14
212 34 -15 -19
324 18 74
corr TU
158 157 99 296
610539 858 668 265 328 313
303P mm
120
44 43 71 56 98 93 24 37 7
14 50
1964
P-Bmm
115 29 13 24 -4 28 32 -29
-19 50
T
"cor*
TU
269
221291 471
1230
970
1715 1640
829
74§_
675 617 !
mm
14 L44
LOO51 139 59 38 87 41 33 8 8
1963
?-Ev cm
9 129 70 4 79
-11 -23
71
* 3
17 3 8
Teorr TU
487 698 693
1240 1805 2690 2745 1815 1730 1050 710
368
p mm
32 29 42 58 143
57
54 96 36 39
57
60 1962
mm
27 14
12 1173 -13 -7 43
2023 52
60
i T
corr TU192 258 136 268 315 538 576 574 590
420472 276
Table II
Precipitation in mm and weighted average in tritium concentration TU, for the years 1962-1966
[
October-December September-March February-February
1965
mm 190 346 733TU
145 188 3501964
mm 207375 625
TU
263 310 825
1963 mm 258 323 720
TU
690 770 950
1962 mm 113 167 592
TU
172 262 4102. ENVIRONMENTAL ISOTOPES IN SURFACE WATER AND GLACIOLOGIC&L STUDIES 2.1 THE USE OF ENVIRONMENTAL ISOTOPES-
PRECIPITATION - INFILTRATION - RUNOFF RELATIONS.
Summary of the paper by T. Dinper
The use of environmental tracers in studying storage and water balance problems in surface and groundwater hydrology is possible only in cases where the tracer concentration in the input to the system is different from the output due to fractionation, radioactive decay and to mixing processes which occur during the storage and the transit of the water in the system. In precipitation - infiltration - runoff relations- the difference between the short-term variations in the
input concentrations and the almost constant tracer concentrations in the output can be used with advantage to separate the different compo- nents of the runoff and to estimate the actual input to the subsurface storage.
It is possible to separate the two basic components of the total runoff by using the equations
- C S Qs + CgQg &> 2>
where Q_ is the total runoff, Q is the surface runoff and Q is the subsurface runoff," and C are the associated environmental tracer co trations. Once the direct runoff is Calculated, infiltration can be estimated from the difference of precipitation and the total runoff
I - P - Q_ (3)
a
The calculation of the infiltration by the subtraction of the total runoff from the precipitation which is used in routine hydrolo- gical studies is thus gredtiy improved, due to the fact that this
latter method does not taTee into account the increase of the subsurface runoff in res-oonse to the increasing infiltration.
It is also Dossible to calculate the build-up of subsurface storage by using the relation
AS - (i - Q_)At (4)
and relate it to Q , thus developing a relation between the active storage and outflow from the storage.
The total subsurface storage volume can be determined using thermonuclear tritium content of the base flow in a given basin. This has been extensively discussed by A. Nir,and some practical applications have already been made. /~lj %J
In studying Precipitation - Infiltratipn - Runoff relations, a good sampling coverage of precipitation and o^ the streamflow is necessary. Samples of groundwater also could be helpful in choosing re-nresentative environmental isotope concentration values for the subsurface runoff. The frequency of sampling depends on the type of problem studied^ in a rainflood study, hourly sampling could be necessary, whereas in a snowmelt runoff study daily sampling would be satisfactory.
2.2 Discussion
The present methods of studying the interaction of precipitation, surface water and groundwater are not adequate. Many of the routine methods are based on some assumptions rather than observations of the hydrologic phenomena. Concepts o^ surface flow, subsurface flow and interflow should be revised in the light of the results of the studies by environmental isotopes.
The study of river basins with environmental isotopes, especially with tritium gives sur-nrising results and shows that only a small portion of water -nreci-nitated during the current year is carried out by the drainage system. A study of environmental isotopes made in a small mountain basin in northern Czechoslovakia [~i "J confirmed this fact.
It was seen that less than kalf of the me&water from the snow appeared in the runoff during the snowmelt season. The larger part of it
infiltrated and reap-neared mixed with water from the preceding and following years, two years later. Jlg.28 gives the basic environmental
isotope and hydrological data in graphical form*
References
/" 1 7 A. Kir, On the interpretation of tritium "age" measurements of groundwater. J. Geophys. Ees. §9_ (1964) No. 12 pp. 2589-95.
/~2 7 T. Dinner and G.H. Davis, Some considerations on tritium dating
*~ ~ and the estimates of the tritium input functions.
Memoires Vol. VIII. International Association of Hydrogeolo gists Congress of Istanbul. 1967 "DP. 2?6-286.
7 T. Dinper et al», Tritium and oxygen-18 in snowmelt-runoff studies (to be^published).
2.3 THE USB OF MVIBOlfMENTAL ISOTOP-S HvT GLACIOLOGICAL STUDIES.
Summary of the contribution by W Dansgaard
Ten years ago Schoellander measured the age of the icebergs coming from glaciers,using the ^4c dating method and found
suprisingly low ages; one was 3000 years old and ten others had ages of less than 1000 years. Recently we repeated the measurements using improved sample collection and extraction techniques. When the snow is accumulated in a glacier, some atmospheric air is trapped in the very fine bubbles which makes the ice- appear white. Since the ice is practically impermeable, the measurement of the *4c content of the air trapped in the ice is a good measure of the period when the air was trapped. To obtain 50 milligrams of carbon for analysis about 4 tons
of ice are required.
Samples were collected from 15 icebergs. The C analyses have not been made yet. but the silicon-32 content of the ice samples has been measured. In fact no silicon-32 has been found in these samples except in one. The measuring accuracy bein 0.01 d m / t o n , which is 30 times lower than the natural specific activity of silicon-32 in the ice, th? ice samples should be at least 3000 years old as one would exnect.
Fig. 29 shows the flow pattern of the ice in Greenland. The ice.
in the central part of the island moves right down to the bottom and then follows a course parallel to the land surface. Since the accumu- lation is 0.3 metres of ice per year and since the depth of the glacier is approximately 3000 m- it -mist take mor'e than 10,000 years for the ice to reach the bottom of the glacier and much more down to the coast.
In 1966, the Cold Region Research Establishment laboratories (CORE) in Hanover, New Hampshire, obtained cores of ice at Camp Century in Greenland down to a depth of 1400 m. The stable isotope variations of these samples were measured and provide an almost unique opportunity to study the past climatic conditions.
The surface velocity of the glacier where the samples were taken is 3«3 m/yr. This implies that all samples which were measured have their origin in the same area.
By studying the stable isotope variation in the core, which shows significant seasonal variations, it is possible to estimate the accumu- lation rate of the ice. The mean oxygen-18 content of the longer
portions of the core, on the other hand, gives information about the mean atmospheric temperature at the time of deposition of the snow several thousand years ago.
The vertical stress in the glacier reduces the wave lengths of the stable isotope content of the ice. Therefore, the actual thickness
is not necessarily the accumulation at the time of deposition.
A correction factor, which depends on the mode of flow of the ice is needed.
Owing to diffusion the amplitude of the seasonal variations is reduced in time. The diffusion coefficient of the ice at prevailing temperatures is 5 x 10~^^ c m2/ s e c Although this value is extremely low, it stabilizes the seasonal variations in several thousand years.
